Delta Drive, Delta Trust & My Drive · Using Delta Drive
What does 'Last heard' mean on a Delta Drive file?
Short answer: Last heard means the Mac holding that offline or cached copy hasn't checked in for a couple of days, so Delta Drive can't confirm the copy is still there. Opening Storage Studio on that Mac updates it.
On the web, the offline and cached marks can show Last heard: in front of them. It means the Mac that holds that copy hasn't checked in for a couple of days.
The file may well still be on that Mac — Delta Drive just can't confirm it right now, and claiming it isn't would be as wrong as claiming it is. Opening Storage Studio on that Mac lets it check in again.
See what the marks mean.